In LTE (Long Term Evolution) Advanced in the 3GPP (Third Generation Partnership Project), a reception technique for user terminals called Interference Rejection Combining is under consideration. The Interference Rejection Combining (IRC) is a technique for downlink communication, in which a user terminal gives weights to signals obtained by respective reception antennas so as to reduce interference by interfering radio wave beams from interfering base stations to the desired radio wave beam from the visited base station (desired base station). The IRC improves the reception quality of desired signals carried over the desired radio wave beam, in particular, when a user terminal 4 is located, as shown in FIG. 1, near the boundary of a visited cell 1a (cell of the desired base station 1) and receives an interfering radio wave beam from another base station 2 (interfering base station) next to the desired base station 1. In FIG. 1, reference numeral 2a denotes the cell of the interfering base station 2. FIG. 1 also shows a schematic shape of a beam 1b generated by the desired base station 1 and a schematic shape of a beam 2b generated by the interfering base station 2. A part of the beam 2b generated by the interfering base station 2, i.e., a part of a beam for downlink channels for other user terminals (e.g., a user terminal 5) causes an interference signal 2c for the user terminal 4.
IRC is described, for example, in Patent Document 1, Non-Patent Document 1, Non-Patent Document 2, and Non-Patent Document 3. In IRC, the user terminal gives weights to each of multiple signals obtained by multiple reception antennas so as to suppress the interference from the interfering radio wave beam to the desired radio wave beam, and the user terminal uses the reception weights to separate the signal destined for the user terminal from signals destined for other user terminals from among the multiple signals derived from radio waves received by the multiple reception antennas. As a type of the IRC, a technique called Successive Interference Cancellation (SIC) has been proposed (e.g., Non-Patent Document 4). In the SIC, the user terminal obtains the desired signal for the user terminal by demodulating an interference signal, which is additionally decoded in some cases, and subtracting the interference signal from the received signal successively.
On the other hand, in recent years, a heterogeneous network (HetNet) has been proposed in which plural types of radio base stations (macro base stations, pico base stations, femto base stations, remote radio heads, etc.) varying in transmission power (transmission capability) are deployed in a multi-layered way (e.g., Non-Patent Document 5).
In the heterogeneous network, base stations having a higher transmission power (transmission capability), e.g., macro base stations, are likely to be selected as the radio access points for user terminals at the stage of cell search or handover, in comparison with other base stations having a lower transmission power (transmission capability), e.g., pico base stations. Accordingly, it is assumed that connections of user terminals are concentrated at base stations with higher transmission power, and thus, there will be a tendency for excessive communication load at base stations with higher transmission power.
Accordingly, a technique called cell range expansion has been proposed. The cell range expansion is a technique to give an offset value (bias value) to the reception quality or reception power from the low-power base station, the reception quality or the reception power being an index for cell selection by the user terminal. The reception quality or the reception power from the low-power base station to which an offset value has been added (or added in the unit of dB) is compared with the reception quality or the reception power from the macro base station. As a result, the reception quality or the reception power from the low-power base station is likely to become better than the reception quality or the reception power from the macro base station. Consequently, since user terminals select to connect to the low-power base station rather than the macro base station, the cell range of the low-power base station is expanded, and it is likely that the communication load of the macro base station will be reduced.
However, when the cell range of the low-power base station is expanded by the cell range expansion (CRE), the user terminal located at the edge of the cell of the low-power base station may be subject to high levels of interference by radio waves from the neighboring high-power base station. Therefore, a technique called enhanced inter-cell interference coordination or enhanced inter-cell interference control, which is an extension of the inter-cell interference coordination or inter-cell interference control, has been proposed. This technique is abbreviated as “eICIC”. The eICIC is described, for example, in Non-patent Document 6.
The eICIC is classified into a frequency domain-based eICIC and a time domain-based eICIC. In either type, eICIC is a technique to limit resources available for a macro base station in order to prevent or minimize interference at user terminals connected with the low-power base station.
In the frequency domain-based eICIC, multiple frequency bands are prepared. First frequency bands are used for downlink transmission from the macro base station to user terminals connected with the macro base station and for downlink transmission from the low-power base stations to user terminals at the centers of the cells of the low-power base stations (e.g., user terminals connected with the low-power base stations without CRE). Second frequency bands are used for downlink transmission from the low-power base station to user terminals at the edges of the cells of the low-power base stations (e.g., user terminals connected with the low-power base stations by virtue of CRE), and are not used for downlink transmission from the macro base station. Thus, it is expected to prevent user terminals at the edges of the cells of the low-power base stations from being interfered with by radio waves from the macro base station.
In the time domain-based eICIC, the macro base station and the low-power base station use the same frequency band, but different time units (for example, subframes) are used for different purposes. The low-power base station is capable of doing continuous downlink transmission. However, the macro base station can perform downlink transmission only intermittently. As a result, a period during which only the low-power base stations perform downlink transmission (protected subframe) and a period during which the macro base station and the low-power base stations perform downlink transmission (non-protected subframe) are repeated. The non-protected subframes are used for downlink transmission from the macro base station to user terminals connected with the macro base station and for downlink transmission from the low-power base stations to user terminals at the centers of the cells of the low-power base stations (e.g., user terminals connected with the low-power base stations without CRE). The protected subframes are used for downlink transmission from the low-power base station to user terminals at the edges of the cells of the low-power base stations (e.g., user terminals connected with the low-power base stations by virtue of CRE). Thus, it is anticipated that it will be possible to prevent user terminals at the edges of the cells of the low-power base stations from interference by radio waves from the macro base station.
Furthermore, another eICIC technique has been proposed in which transmission from the high-power base station is permitted but the transmission power is limited to be low at the resources at which the low-power base stations execute transmission for user terminals at the edges of the cells of the low-power base stations (Non-patent Document 7). Specifically, in the frequency domain-based eICIC, even the above-mentioned second frequency bands are used for downlink transmission from the macro base station, but the transmission power is limited to be low, so that interference at user terminals at the edges of the cells of the low-power base stations is restricted. In the time domain-based eICIC, even the protected subframes are used for downlink transmission from the macro base station, but the transmission power is limited to be low, so that interference at user terminals at the edges of the cells of the low-power base stations is restricted.